The Journey to the Top Quark

May 04, 1994|By BRUCE BARNETT

Mankind's journey to the announcement last week of evidence for the existence of the top quark began long ago. Cave men probably gazed at sky or meadow and wondered, ''Why does this exist? How does it work? Why are we here?''

Slowly, science found answers. Atoms make molecules; nuclei and electrons make atoms; protons and neutrons make nuclei.

Thirty years ago, Murray Gell-Mann suggested that particles like protons and neutrons are made of little things called ''up and down quarks.'' They have electric charges of 2/3 and - 1/3 of the proton's charge. There is also a third quark, the ''strange'' quark, again with - 1/3 charge. The three quarks and four other small particles called leptons (the electron, muon and two neutrinos) were hypothesized to be the fundamental building blocks of nature. Gell-Mann won the Nobel prize for this idea.

In November, 1974, a revolution occurred. Two experiments, one led by Burton Richter operating at the Stanford Linear Accelerator Center in California and another led by Sam Ting at the Brookhaven National Laboratory on Long Island, claimed to discover another quark, the ''charm'' quark. It had charge 2/3 and a mass nearly twice that of a proton. Richter and Ting shared the Nobel Prize for that discovery.

There was suddenly a peculiar symmetry: 4 quarks and 4 leptons. The symmetry went further. Each set of four was actually made of two sets of two: the electron and its neutrino, the muon and its neutrino, the up and down quarks, the charm and strange quarks. Why are groupings of quarks and leptons similar? No one knew.

In 1975 a member of Richter's team, Martin Perl, discovered another lepton, the tau, and asserted there was another neutrino.

The tau destroyed the quark-lepton symmetry. Mankind had 4 quarks and 6 leptons. Did that mean there should be 2 more quarks?

In 1977, a fifth quark, the bottom quark, was discovered in an experiment led by Leon Lederman at Fermilab near Chicago. Its charge was - 1/3 and its mass was more than five times that of the proton. Five quarks and 6 leptons. Surely, there must be another quark out there -- the top.

Higher-energy accelerators were built at Stanford and in Germany. Theoreticians guessed that they would produce the top quark. They were wrong. The new machines produced many new measurements on charm and bottom. We proved that the particle responsible for the force between quarks, the gluon, existed. But we found no top.

Still higher energy accelerators were begun at Stanford and in Switzerland. Again the top quark eluded discovery. Both machines were designed to study a particle called the ''Z,'' which, along with another particle called the ''W,'' is responsible for the ''weak'' force, which causes some types of nuclear radioactive decays.

Some particles make up matter, while others supply forces. The matter-making particles, quarks and leptons, interact with one another through forces caused by the exchange of other particles, the Z, W, gluons and photons. The photon causes the electric and magnetic forces.

It is like baseball, football or tennis, where players interact through the exchange of baseballs, footballs or tennis balls. The players are the quarks and leptons. The balls are the Z, W, gluons and photons.

In September 1989 scientists at Stanford announced the first results. We found no evidence for top, but we found something else of fantastic significance. We showed with 20-1 betting odds that there were only three neutrinos, which probably meant that there were only six leptons. From symmetry that meant there were probably only six quarks. The top quark might be the last missing fundamental building block.

Only a month later physicists in Switzerland confirmed these results and quickly improved the betting odds to more than a million to one through a series of extremely precise experiments.

In 1989 the world's highest-energy accelerator was the Tevatron at Fermilab, outside Chicago. It collides protons into antiprotons 250,000 times per second. An experiment there, CDF, proved that top quarks must have a mass at least 100 times that of a proton.

Several experimental upgrades were made to the CDF detector, and a new data run began in 1992. By last summer, a trillion proton-antiproton collisions occurred within CDF. More than 16 million of these events were written onto magnetic tape. Last week, using three different search methods, CDF announced that 12 of these events gave evidence that the top quark exists.

The CDF collaboration includes 440 physicists, 8 of them from Johns Hopkins University. We at Hopkins, with a few of the other CDF members, were responsible for one of the CDF upgrades. Our apparatus, a ''Silicon Vertex Detector,'' or SVX, was critical in the identification of 7 of these 12 events. Without SVX, no announcement of the top quark would have been made.